1. Field of the Invention
The present invention relates to devices which are used for measuring the density of members, such as bones, and more particularly to devices which utilize ultrasonic acoustic signals to measure the physical properties and integrity of the members.
2. Description of the Prior Art
The present invention relates to bone measuring instruments and in particular to instruments employing ultrasonic pulses to make measurements of the integrity of human bones in vivo.
Osteoporosis or loss of bone mineralization and its cure or prevention are important areas of medical and biological interest. Of particular concern is the loss of trabecular bone, a spongy bone structure forming the interior of vertebrae and other bones. The trabecular bone provides much of the strength of such bones and is disproportionately affected in times of bone mass loss. While it has long been known that the speed of sound through a material will reveal properties of that material, application of this principle for reliable clinically significant bone density measurement has not been easy.
An early measurement of sound speed through in vitro bone is described in: Sonic Measurement of Bone Mass, Clayton Rich et al., a paper delivered at a conference held in Washington, D. C. Mar. 25-27, 1965, NASA publication SP-64. A sinusoidal pulse was timed in its passage from a transmitting transducer through a water bath and excised bone to a receiving transducer. The author noted problems of providing sufficient signal strength in trabecular bone even with the use of an automatic gain control circuit.
Sound speed measurement of bone in vivo is described in U.S. Pat. No. 3,847,141 to Hoop issued Nov. 12, 1974. A pulse from a transmitting transducer was propagated through a finger to be received by a receiving transducer; however, the pulse was not timed directly. Rather after filtering, the pulse was used to xe2x80x9cretriggerxe2x80x9d the transmitting transducer to create a regular series of pulses whose frequency could be determined. This technique is known generally as xe2x80x9csing aroundxe2x80x9d.
A doctoral thesis by Langton entitled xe2x80x9cThe Measurement of Broadband Ultrasonic Attenuation in Cancellous Bonexe2x80x9d dated July 1984 describes the measurement of the speed of sound through the os calcis of the heel. The author, however, found that accurate measurement of sound speed was hampered by the difficulty of measuring the beginning of the pulse on an oscilloscope and suggested that the elapsed time between the transmission and reception of a pulse was too short for accurate and repeatable measurements. He proposed that the xe2x80x9csing aroundxe2x80x9d approach of Hoop might be used to correct this latter problem. Langton, observing the considerable frequency distortion of the pulse after passage through the heel, elected to continue his investigation in the area of frequency dependant attenuation rather than sound speed.
The present invention provides significantly improved accuracy in speed of sound measurements of bone in vivo. The present inventors have recognized that prior-art detection systems, using analog threshold detection techniques, were susceptible to timing variations caused by phase and amplitude distortion of the transmitted pulse. Even small variations in detection time of a pulse traveling across the narrow width of the human heel, for example, can produce unacceptable variations in sound speed determination.
The present invention reduces variation in detection time by the extensive application of digital control techniques across the entire signal chain. The received signal is digitized to be received by a microprocessor allowing flexible numerical analysis of pulse arrival time tailored to the particular device, frequency range, and region of investigation. Conversion of the received pulse into digital words for receipt by the microprocessor, together with initialization of the transmitted pulse by the microprocessor allows computer-stable timing of the transmission of the pulse. The microprocessor adjusts the transmitted pulse strength and the gain of the receiver amplifier to dynamically optimize signal strength. Digitization of the received pulse also allows a single captured pulse to be used for both the purpose of measuring velocity or time of flight of the ultrasonic signal and in analyzing its attenuation through mathematical techniques such as the fast Fourier transform.
In combination these techniques rendered possible clinically accurate speed of sound measurements of bone in vivo.
More specifically, the present invention provides an ultrasonic densitometer for measurement of the human os calcis in vivo including an ultrasonic signal generator producing a broad band electrical pulse of ultrasonic frequencies and a first ultrasonic transducer connected to the ultrasonic signal generator for producing a corresponding acoustic signal directed along a transmission axis.
A second ultrasonic transducer receives the acoustic signals directed along the transmission axis and relays them to an analog to digital converter converting the electrical signals to digital representations. A microprocessor communicating with the ultrasonic signal generator and the analog to digital converter executes a stored program to initiate the transmission of the acoustic pulse and to numerically analyze the digital representation of the received acoustic signal as distorted by the imposition of the human heel between the first and second ultrasonic transducers to measure a time of transmission of the ultrasonic pulse between the first and second transducers.
Thus it is one object of the invention to provide improved accuracy in the measurement of time of transmission through the use of numerical analysis which may accommodate for pulse distortion and noise.
It is another object of the invention to provide for the digital control by a single microprocessor in both initiation of the transmission of the acoustic pulse and its receipt and analysis such as provides more precise measurement.
The ultrasonic signal generator may be a digitally controllable amplifier designed to create a pulsed output.
Thus it is another object of the invention to provide control by the microprocessor of the output signal as well as the processing of the received signal.
The densitometer may include a digitally controllable automatic gain control circuit connected between the second ultrasonic transducer and the analog to digital converter to receive the electrical signal from this second ultrasonic transducer and receive a control signal from the microprocessor. The microprocessor operating according to its stored program may control the amplification of the electronic signal from the second ultrasonic transducer prior to its receipt by the digital to analog converter.
Thus it is another object of the invention to provide precise digital gain control as may be necessary to optimize the sensitivity of the receive transducer and the amplifier circuit to received acoustic signals.
It is yet another object of the invention to provide for a conversion of the received signal to a digital form without loss of resolution by controlling the gain to fully use the range of the A to D converter.
The microprocessor may further numerically analyze the digital representation of the received acoustic signal as distorted by the human heel to measure a change in shape of the waveform.
Thus it is another object of the invention to provide an extremely more accurate machine that provides both a measurement of transit speed of the ultrasonic wave and its attenuation such as may provide alternative views of the bone integrity or which may be combined to provide a more robust measurement of bone integrity. It has been determined that these two measurements supplement each other.
The measurement of the time of flight of the pulse and the measurement of pulse shape may be in comparison to previously measured a standard.
Thus it is another object of the invention to simplify the comparison of measurements to a standard by use of a microprocessor which may store earlier and later measurements.
The densitometer may include a digital display communicating with the microprocessor and the microprocessor may execute a stored program to provide data to that display indicating the physical property of the os calcis of the heel.
Thus it is another object of the invention to provide a densitometer having suitable accuracy for a digital display that provides a simple, quantitative and unambiguous measurement of bone integrity that may not be obtained from visual display of waveforms or frequency measurements in a sing around system.
The foregoing and other objects and advantages of the invention will appear from the following description. In this description, reference is made to the accompanying drawings which form a part hereof and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention.